Recently, a nickel-phosphorous alloy has been expected as an alternative candidate of hexavalent chrome compounds because of good properties for anti-corrosion, anti-wear, and hardness. On the other hand, the composition of nickel and phosphorous within the deposit depends on the electroplating condition. Multilayer films in which the composition modulates can be fabricated by controlling current density and cathodic potential. Multilayer films are expected to have the advantages of the properties in comparison with single-layer films. The multilayer films deposited by the electroplating technique were studied by using XRD, the depth profiling based on AES and XPS, and the Vicker's hardness.
The nickel-phosphorous multilayer films were deposited from a single bath by pulse plating. The bath consisted of 0.5 M nickel sulfate, 0.13 M nickel chloride, 0.1 M citric acid, and 0.05 M phosphorous acid. The bath was kept at temperature of 50 °C, and pH of 1.5. For constant cathodic potential, the high (low) phosphorous content layer was deposited with current density of 5 (200) mA/cm2. High current density produced a crystalline layer with the phosphorous content of 4 wt%. While, low current density produced an amorphous layer with 13 wt%. The depth profiling was performed by AXIS ULTRA DLD (Kratos Inc.) with the argon ion sputtering.
A single-layer film with a thickness of 20 µm, and multilayer films that consist of each layer thickness of 5 µm, 1 µm, 100 nm, 50 nm, 25 nm, and 10 nm were examined. Total film thickness of the multilayer films was equal to a thickness of the single-layer film. The Vicker's hardness of the multilayer films indicated higher values than that of the single-layer film. Furthermore, the hardness increased at the multilayer films fabricated by the thinner layer. It is considered that small grain size results an increase of the hardness at the thin-layer film. Depth profiling revealed a periodic content change of nickel and phosphorous respect to the sputtering time. It was confirmed that the high and low phosphorous content layer appeared by turns even at the multilayer films with a layer thickness of nanoscale. |